I was able to replicate this issue from my side. This error appears only for Stamps.com shipping integration and only printing integrated invoice with label on it. Our development team is investigating this issue and will update you in a support ticket as soon as possible. I apologise for the inconvenience.
I am not able to reprint shipping labels on open orders or processed orders. Is anybody facing similar issues? I am getting this " this is not a valid pdf file"
Have a ticket open with LW since 3 weeks but not getting any real answer